So, you have the money to replace only one main fixture in the bathroom. Is it going to be the toilet, or the tub? When you look at things from the viewpoint of a plumber and plumbing services, there is one fixture that clearly comes out on top. Based on the following information, you should be able to figure out which fixture is easier, faster, and cheaper to install.
Installing a Toilet
To install a toilet, you first decrease the level of water in the bowl. You can either chase the water down with a bucket of water, or you can flush and then use the valve behind the toilet to turn off the water so that it cannot refill the bowl. Next, detach the tank from the bowl component, and then detach the valve and water line from the tank. Lots of towels or other absorbent products should be present, as this will get very wet, very fast.
Your plumber will need to take the tank outside so that it will not continue to leak water everywhere. Then he/she will unscrew the bowl from the floor and twist it to the side to release the O-ring/wax ring underneath that holds and seals the toilet bowl to the top of the toilet stack pipe. It is going to get very messy and smelly at this point because the sewer pipe will be exposed. The bowl also has to be moved outside as it will continue to drain remaining water through the trap in the base of the toilet. Finally, all of these steps are reversed when installing the new toilet. The whole process should take about an hour or less, unless the plumber encounters some irregularities with the O-ring/wax ring underneath the toilet or rot in the floor boards under the toilet.
Installing a New Tub
Installing a new tub will take at least a day or more. That is because the old tub has to be broken up and taken out in as many large pieces as possible. Exposing the walls and floor may reveal underlying issues that may need remediation. Then there is the tile or tub surround walls, all of which will have to be removed and replaced. It involves a completed demolition/deconstruction before any new tub and walls can be installed. If you want any changes made to the size, shape, and location of the tub, or you want the plumbing fixtures to be oriented another way from what you have, that will lengthen the time it takes to complete a tub install.